Grass–legume mixtures are key crops to improve agricultural sustainability. Despite their significant use in mixture, lucerne (Medicago sativa L.) varieties are mostly tested and bred for monocultures. This study was performed to evaluate whether the ranking of lucerne genotypes for biomass components and quality was similar in monoculture and mixture, and whether traits recorded on isolated plants could help to predict performance in monoculture and mixture. For 46 genotypes planted in the three competition conditions, plant biomass, shoot height, shoot number and protein content were recorded. In addition, maximum leaf size, internode length, stem diameter, growth habit, specific leaf area and leaf dry-matter content were measured on isolated plants. A general positive correlation was observed between the performance of genotypes in monoculture and in mixture. However, significant changes in genotype ranking indicated that the species of its neighbours could modify the relative performance of a genotype. The traits that minimised competition intensity also changed according to the neighbour species. In mixture, competition intensity was highest for plants with long internodes, a high shoot number and a non-erect growth habit. In monoculture, plants with long internodes and larges leaves suffered less from competition. The agronomic performance of lucerne varieties differing in their architecture should be compared in monoculture and mixture to finally identify the traits to be phenotyped to improve lucerne variety performance in both cultivation modes.